This invention relates to a water jet propulsion unit for use in water-borne craft,
Water jet propulsion units have hitherto been of two main types: those in which axial flow is achieved by the use of stators or straightening vanes, being of either axial or mixed-flow configuration; and those in which axial flow is achieved with or without stator assistance, by the use of counter-rotating impellers, also being of axial of mixed-flow configuration.
The use of counter-rotating impellers is described in our PCT application WO94/08845 whereby large efficiency gains are achieved by providing a low pressure, high mass design, utilising a pair of in-line impellers fixed to co-axial shafts and a nozzle throttling device. In the designs described in WO94/08845 the radial component issuing from the upstream impeller is eliminated by mechanically driving the down-stream impeller, which is of opposite but similar pitch, in the opposite direction.
The prior art references in WO94/08845 all describe two in-line impellers counter-rotating about the same axis.
One disadvantage of in-line impellers is that the driving means for such impellers necessarily require either complex gearing in a common housing such as in DE 3,942,672 or concentric shafts such as are shown in WO94/08845. It would be desirable to be able to provide side by side impellers having separate driving shafts with more straight forward bearing mountings than with the in-line configuration.
In order to increase the rate of flow through a jet propulsion unit which has axial in line impellers it is necessary either to increase the speed of rotation of the impellers or else to increase their diameter. It would be advantageous to be able to increase the rate of flow without doing either.
It is an object to go some way towards achieving these desiderata or at least to offer the public a useful choice.
Accordingly the invention may be said broadly to consist in a water jet propulsion unit comprising:
an intake section,
a pump section having a pair of separate cylindrical passages therethrough, and
a mixing/discharge section ending in a discharge nozzle,
the sections being in smooth commnunication with each other,
a pair of impellers in said pump section rotatable in opposite directions on parallel axes in a substantially side by side configuration, one within each said cylindrical passage,
said mixing /discharge section being adapted to converge the flows of water being discharged from said impellers in a manner which substantially neutralises non-axial flow components from one said impeller by means of non-axial flow components from the other said impeller and maximises axial flow out of said mixing/discharge section.
Preferably said impellers are axial flow impellers.
In one embodiment said mixing/discharge section is substantially oval in cross-section at its plane of intersection with said pump section and converges in a downstream direction to be substantially circular in cross-section at said nozzle.
The term xe2x80x9covalxe2x80x9d for the purposes of this specification means a closed geometric figure consisting of opposed half circles of equal radii joined by parallel straight lines tangential to each said half circle.
Preferably there is provided at the upstream end of said mixing/discharge section a common wall between the streams of water discharged from said pump section, the sides of said common wall converging into an opening beginning at a point intermediate said parallel axes and extending in a downstream direction in smooth curves to intersect with opposite walls of said mixing/discharge chamber at points in the same plane orthogonal to said parallel axes.
In one embodiment said intake section is divided into sections, one section leading to each separate passage through said pump section.
Preferably, when said intake section is so divided, there is provided a flow deflecting means in said intake chamber to equalise the flow of water entering each of said separate passages through said pump section.
In one embodiment said impellers are mounted on drive shafts to be driven by a single driving means.
In another embodiment said impellers are mounted on drive shahs to be driven by separate driving means.
Preferably the cross-sectional area of said discharge nozzle is adjustable.
Preferably the blades of said impellers are of equal but opposite pitch.
Preferably the peripheral blade angles of said impellers are in the range of about 25xc2x0 to about 40xc2x0.
In one embodiment the unit is calibrated to operate in a high pressure/low mass mode.
In another embodiment the unit is calibrated to operate in a low pressure/high mass mode.
In another alternative embodiment there are two pairs of separate passages through said pump section and two pairs of counter-rotating side by side impellers mounted therein.
In one embodiment said intake section is divided into four sections, one section leading to each separate passage through said pump section.